ACPAtmospheric Chemistry and PhysicsACPAtmos. Chem. Phys.1680-7324Copernicus PublicationsGöttingen, Germany10.5194/acp-10-6993-2010Oxidative capacity of the Mexico City atmosphere – Part 2: A RO<sub>x</sub> radical cycling perspectiveSheehyP. M.12VolkamerR.134MolinaL. T.12MolinaM. J.131Massachusetts Institute of Technology, Cambridge, MA, USA2Molina Center for Energy & the Environment, La Jolla, CA, USA3University of California at San Diego, La Jolla, CA, USA4University of Colorado at Boulder and CIRES, Boulder, CO, USA30072010101469937008This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from http://www.atmos-chem-phys.net/10/6993/2010/acp-10-6993-2010.htmlThe full text article is available as a PDF file from http://www.atmos-chem-phys.net/10/6993/2010/acp-10-6993-2010.pdf

A box model using measurements from the Mexico City Metropolitan Area study
in the spring of 2003 (MCMA-2003) is presented to study oxidative capacity
(our ability to predict OH radicals) and RO<sub>x</sub>
(RO<sub>x</sub>=OH+HO<sub>2</sub>+RO<sub>2</sub>+RO) radical cycling in
a polluted (i.e., very high NO<sub>x</sub>=NO+NO<sub>2</sub>) atmosphere.
Model simulations were performed using the Master Chemical Mechanism
(MCMv3.1) constrained with 10 min averaged measurements of major radical
sources (i.e., HCHO, HONO, O<sub>3</sub>, CHOCHO, etc.),
radical sink precursors (i.e., NO, NO<sub>2</sub>, SO<sub>2</sub>,
CO, and 102 volatile organic compounds (VOC)), meteorological
parameters (temperature, pressure, water vapor concentration, dilution), and
photolysis frequencies.
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Modeled HO<sub>x</sub> (=OH+HO<sub>2</sub>) concentrations compare
favorably with measured concentrations for most of the day; however, the
model under-predicts the concentrations of radicals in the early morning.
This "missing reactivity" is highest during peak photochemical activity,
and is least visible in a direct comparison of HO<sub>x</sub> radical
concentrations. We conclude that the most likely scenario to reconcile model
predictions with observations is the existence of a currently unidentified
additional source for RO<sub>2</sub> radicals, in combination with an additional
sink for HO<sub>2</sub> radicals that does not form OH. The true
uncertainty due to "missing reactivity" is apparent in parameters like
chain length. We present a first attempt to calculate chain length rigorously
i.e., we define two parameters that account for atmospheric complexity, and
are based on (1) radical initiation, <i>n</i>(OH), and (2) radical
termination, ω. We find very high values of <i>n</i>(OH) in the early morning
are incompatible with our current understanding of RO<sub>x</sub> termination
routes. We also observe missing reactivity in the rate of ozone production
(<i>P</i>(O<sub>3</sub>)). For example, the integral amount of ozone produced could be
under-predicted by a factor of two. We argue that this uncertainty is partly
accounted for in lumped chemical codes that are optimized to predict ozone
concentrations; however, these codes do not reflect the true uncertainty in
oxidative capacity that is relevant to other aspects of air quality
management, such as the formation of secondary organic aerosol (SOA). Our
analysis highlights that apart from uncertainties in emissions, and
meteorology, there is an additional major uncertainty in chemical mechanisms
that affects our ability to predict ozone and SOA formation with confidence.